Gradient coil apparatus and method of fabricating a gradient coil to reduce artifacts in MRI images

ABSTRACT

A gradient coil or other element of a magnetic resonance imaging system includes at least one layer comprised of copper having a first surface and a second surface. A first semiconductor layer is applied to the first surface of the copper and an insulation layer applied to the first semiconductor layer. In one embodiment, a second semiconductor layer is applied to the second surface of the copper. The first and second semiconductor layers encapsulate any voids formed between the copper and the semiconductor layers, equalize the potential around the voids and prevent partial discharge formation.

FIELD OF THE INVENTION

The present invention relates generally to magnetic resonance imaging(MRI) systems and in particular to a gradient coil and a method offabricating a gradient coil to reduce artifacts in MRI images.

BACKGROUND OF THE INVENTION

Magnetic resonance imaging (MRI) is a medical imaging modality that cancreate pictures of the inside of a human body without using x-rays orother ionizing radiation. MRI uses a powerful magnet to create a strong,uniform, static magnetic field (i.e., the “main magnetic field”). When ahuman body, or part of a human body, is placed in the main magneticfield, the nuclear spins that are associated with the hydrogen nuclei intissue water become polarized. This means that the magnetic moments thatare associated with these spins become preferentially aligned along thedirection of the main magnetic field, resulting in a small net tissuemagnetization along that axis (the “z axis,” by convention). An MRIsystem also comprises components called gradient coils that producesmaller amplitude, spatially varying magnetic fields when a current isapplied to them. Typically, gradient coils are designed to produce amagnetic field component that is aligned along the z axis, and thatvaries linearly in amplitude with position along one of the x, y or zaxes. The effect of a gradient coil is to create a small ramp on themagnetic field strength, and concomitantly on the resonant frequency ofthe nuclear spins, along a single axis. Three gradient coils withorthogonal axes are used to “spatially encode” the MR signal by creatinga signature resonance frequency at each location in the body. Radiofrequency (RF) coils are used to create pulses of RF energy at or nearthe resonance frequency of the hydrogen nuclei. The RF coils are used toadd energy to the nuclear spin system in a controlled fashion. As thenuclear spins then relax back to their rest energy state, they give upenergy in the form of an RF signal. This signal is detected by the MRIsystem and is transformed into an image using a computer and knownreconstruction algorithms.

Each gradient coil used in an MRI system, or other elements in the MRIsystem, may consist of a plurality of layers including conducting coppersheets or boards and insulation layers (e.g., an epoxy resin). FIG. 1 isan exemplary prior art laminate stack for a gradient coil. The gradientcoil stack 100 includes a copper sheet or board 102, a laminate 106(e.g., a fiberglass substrate) that is used as a backing for copper 102,and an insulation coating such as epoxy resin 104. The copper sheet 102may be etched with a pattern or trace (e.g., a “fingerprint” pattern).Various processes, such as lamination or vacuum pressure impregnation(VPI), may be used during the manufacture or fabrication of the gradientcoil 100. For example, in one fabrication process, the copper sheet 102is laminated directly to the laminate 106. The entire board may then beimpregnated with epoxy resin 104 using a VPI process. For example, theresin 104 may be injected into a mold (while the mold is under a vacuum)that forms the shape of the gradient coil.

During fabrication of the gradient coil, voids may form at an interface120 between the copper 102 and the resin 104, at an interface 122between the copper 102 and the laminate 106, in the resin 104, or in thelaminate 106. For example, a void 108 is shown in FIG. 1 in the resin104 at the interface between the copper 102 and resin 104. There areseveral possible sources of void formation in a gradient coil. Onesource of void formation is poor bonding/bonding strength between thecopper 102 and the epoxy resin 104 or between the copper 102 and thelaminate 106. For example, typically the epoxy resin does not bond wellto an inorganic metal. Another source of void formation are leaks in themold used for vacuum pressure impregnation of the epoxy resin 104. Leaksin the mold may form bubbles inside the VPI epoxy resin. In addition,incomplete coverage (e.g., delamination) during pressing of the copper102 and laminate 106 is another possible source of voids.

Any voids that form in the gradient coil, e.g., void 108 shown in FIG.1, are susceptible to partial discharges. An electric field is inducedin a void and at a partial discharge inception voltage (PDIV), thevoltage difference across the void causes a small spark (or partialdischarge) to bridge the void. The spark or partial discharge causes aradio frequency (RF) noise burst to be emitted. The MRI system candetect the RF noise burst which will cause artifacts in the MRI imagegenerated by the MRI system. For example, a partial discharge in agradient coil may yield an effect in k-apace known as a “white pixel.”The “white pixel” produces an artifact in the reconstructed MR imagemaking an image undesirable and difficult to interpret.

Accordingly, there is a need for a gradient coil and method offabricating a gradient coil that reduces the number of voids and reducesor eliminates the partial discharge in voids that are formed in thegradient coil. In addition, it would be desirable to provide a gradientcoil and method of fabricating a gradient coil that improves the bondingof the copper to an insulation layer and the bonding of the copper andto the fiberglass substrate. It would be also advantageous to provide amethod of eliminating the field generation across a void in an MRIsystem by equalizing the potential difference across the void.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with an embodiment, a gradient coil for a magneticresonance imaging system includes at least one layer comprised of acopper sheet having a first surface and a second surface, a firstsemiconductor layer applied to the first surface of the copper sheet,and an insulation layer applied to the first semiconductor layer.

In accordance with another embodiment, a method for fabricating agradient coil for a magnetic resonance imaging system, the methodincludes applying a first semiconductor layer to a first surface of acopper sheet, applying a second semiconductor layer to a second surfaceof the copper sheet, applying a fiberglass substrate layer to the secondsemiconductor layer, and applying an insulation layer to the firstsemiconductor layer.

In accordance with another embodiment, a magnetic resonance imagingsystem includes at least one copper surface, at least one semiconductorlayer applied to the at least one copper surface and an insulation layerapplied to the at least one semiconductor layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more fully understood from the followingdetailed description, taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is an exemplary prior art laminate stack for a gradient coil.

FIG. 2 illustrates a method of fabricating a gradient coil in accordancewith an embodiment.

FIG. 3 is a schematic block diagram of a gradient coil laminate stack inaccordance with an embodiment.

FIG. 4 is a schematic block diagram of a gradient coil laminate stackincluding a void in accordance with an embodiment.

DETAILED DESCRIPTION

FIG. 2 illustrates a method of fabricating a gradient coil in accordancewith an embodiment. At block 202, the fabrication process begins. Atblock 204, a copper sheet or board is machined. During machining, thecopper sheet may be etched to include a pattern or trace, for example, a“fingerprint” trace. At block 206, the copper may be sanded to roughenthe surface of the copper to promote adhesion. At block 208, the coppermay be cleaned to remove, foe example, grease and dust. At block 210, afirst layer of a semiconductor material is applied to a first surface ofa copper sheet or board. For example, the semiconductor may be appliedto a top surface of the copper sheet. In alternative embodiments, thecopper may be in the form of a solid round copper conductor, a hollowconductor, or wire(s). In such embodiments, the semiconductive materialmay be applied as a coating or as a tape wrapped around the copperconductor. At block 212, a layer of the semiconductor material isapplied to a second surface of the copper sheet or board. For example,the semiconductor may be applied to a bottom surface of the coppersheet. The semiconductor may be, for example, conductive epoxy black orother semiconductor material. Alternatively, a conductive epoxy, metalfilled resin (e.g., a metal filled polymeric resin) or other materialsthat are poor conductors may be used instead of the semiconductor. Inone embodiment, the semiconductor may be laminated to the first andsecond surface of the copper sheet. Methods of lamination generallyknown in the art may be used to apply the semiconductor layers.Alternatively, the semiconductor may be applied, for example, as a woundtape, by solution coating, or as an adhesive tape.

At block 214, a laminate backing is laminated to a layer of thesemiconductor, for example, to the semiconductor layer applied to thebottom surface of the copper sheet. The laminate and semiconductor layerare bonded at a first laminate surface, e.g., a top surface of thelaminate. The laminate may be, for example, an FR4 fiberglass substrate,plastic, teflon, etc. In another embodiment, a third semiconductor layermay be applied (e.g., laminated) to a second laminate surface, e.g., abottom surface of the laminate. Methods of lamination generally known inthe art may be used to apply the laminate.

FIG. 3 is a schematic block diagram of a gradient coil laminate stack inaccordance with an embodiment. In FIG. 3, a first semiconductor layer314 is applied to a first surface 316 of a copper sheet 302 and a secondsemiconductor layer 312 is applied to a second surface 318 of the coppersheet 302. A laminate 306 (e.g., a fiberglass substrate) is laminated,for example, a first laminate surface 326, to the second semiconductorlayer 312. The first and second semiconductor layers 312, 314 arepreferably epoxy doped or an organic coating so that it will bond wellto the epoxy resin used to encapsulate the gradient coil laminate board300 as described further below. Improved bonding may prevent theformation of voids. In addition, any voids that do form between thecopper surface and the semiconductor will be contained within thesemiconductor layer. In another embodiment, a third semiconductor layer(not shown) may be applied (e.g., laminated) to a second laminatesurface 328. The third semiconductor layer (not shown) is electricallycoupled to the copper 302, for example, the third semiconductor layermay be wrapped around at the ends to be coupled to the copper. Voidsthat form between the semiconductor and the laminate or in the laminateare encapsulated by the semiconductor.

As discussed above, in alternative embodiments, the copper conductor 302may be a solid round copper conductor, a hollow conductor, or wire(s). Asemiconductor layer may be applied as a coating or as a semiconductortape wrapped around the copper conductor. In addition, a conductiveepoxy or metal filled resin may be used instead of a semiconductormaterial.

Returning to FIG. 2, at step 216, an insulation layer, e.g., an epoxyresin is applied to the gradient coil laminate stack. In one embodiment,the epoxy resin may be applied using vacuum pressure impregnation (VPI).VPI methods generally known in the art may be used to apply the epoxyresin. In alternative embodiments, the epoxy resin may be applied usingresin infusion molding, resin transfer molding, vacuum assisted resintransfer molding or lamination. A laminate stack including the epoxyresin is shown in FIG. 4.

FIG. 4 is a schematic block diagram of a gradient coil laminate stackincluding a void in accordance with an embodiment. As mentioned, asemiconductor layer 414 is applied to a first surface 416 of the copper402 before application of an epoxy resin 404 or other insulation layer(e.g., by VPI). The semiconductor layer 414 is located between thecopper 402 and the resin 404. As discussed above with respect to FIG. 2,a second semiconductor layer (not shown) may be applied to a secondsurface 418 of the copper 402. Any voids, e.g., void 408, that formbetween the copper 402 and the semiconductor 414 during fabrication(e.g., during a lamination process) are contained within thesemiconductor layer 414. Semiconductor 414 encapsulates void 408 andacts as an equipotential surface. The semiconductor 414 will keep thevoid at the same potential (i.e., the semiconductor will equalize thepotential around any voids, e.g., void 408, between the copper 402 andthe semiconductor 414) due to the small conductivity of thesemiconductor. Accordingly, an electric field will not form across thevoid 408 preventing a partial discharge from occurring in void 408. Byapplying a semiconductor layers or layers to the surface(s) of thecopper sheet, any voids formed between the copper and the semiconductorwill be encapsulated in an equipotential volume which will eliminate orreduce the potential difference across the void and prevent partialdischarge formation. In addition, by applying a semiconductor layers orlayers to the surface(s) of the laminate (not shown), for example, afiberglass substrate, any voids formed between the laminate and thesemiconductor will be encapsulated in an equipotential volume which willeliminate or reduce the potential difference across the void and preventpartial discharge formation.

In other embodiments, a semiconductor layer or layers may be applied toother copper surfaces in the MRI system that are insulated with amaterial such as epoxy resin to reduce or prevent partial dischargeformation. The reduction or elimination of partial discharges will inturn reduce or prevent artifacts being generated in an MRI image.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to make and use the invention. The patentable scope of the inventionis defined by the claims, and may include other examples that occur tothose skilled in the art. Such other examples are intended to be withinthe scope of the claims if they have structural elements that do notdiffer from the literal language of the claims, or if they includeequivalent structural elements with insubstantial differences from theliteral language of the claims. The order and sequence of any process ormethod steps may be varied or re-sequenced according to alternativeembodiments.

Many other changes and modifications may be made to the presentinvention without departing from the spirit thereof. The scope of theseand other changes will become apparent from the appended claims.

1. A gradient coil for a magnetic resonance imaging system, the gradientcoil comprising: at least one layer comprised of copper having a firstsurface and a second surface; a first semiconductor layer applied to thefirst surface of the copper; and an insulation layer applied to thefirst semiconductor layer.
 2. A gradient coil according to claim 1,wherein the copper is formed as one of a sheet, a solid round conductor,a hollow conductor or a wire.
 3. A gradient coil according to claim 1,wherein the copper comprises an etched pattern on the first surface ofthe copper.
 4. A gradient coil according to claim 1, further comprising:a second semiconductor layer applied to the second surface of thecopper.
 5. A gradient coil according to claim 4, further comprising: alaminate layer applied to the second semiconductor layer.
 6. A gradientcoil according to claim 5, wherein the laminate layer is laminated tothe second semiconductor layer.
 7. A gradient coil according to claim 5,wherein the laminate layer has a first laminate surface and a secondlaminate surface and the first laminate surface is coupled to the secondsemiconductor layer, the gradient coil further comprising: a thirdsemiconductor layer applied to the second laminate surface of thelaminate layer and electrically coupled to the copper.
 8. A gradientcoil according to claim 4, wherein the second semiconductor layer islaminated to the second surface of the copper.
 9. A gradient coilaccording to claim 1, wherein the insulation layer is an epoxy resin.10. A gradient coil according to claim 1, wherein the firstsemiconductor layer is laminated to the first surface of the copper. 11.A gradient coil according to claim 1, wherein the insulation layer isvacuum pressure impregnated.
 12. A gradient coil according to claim 1,wherein the first semiconductor layer is applied as a wound tape.
 13. Amethod for fabricating a gradient coil for a magnetic resonance imagingsystem, the method comprising: applying a first semiconductor layer to afirst surface of a copper layer; applying a second semiconductor layerto a second surface of the copper layer; applying a laminate layer tothe second semiconductor layer; and applying an insulation layer to thefirst semiconductor layer.
 14. A method according to claim 13, whereinapplying the first semiconductor layer to a first surface of the copperlayer comprises laminating the first semiconductor layer to the firstsurface of the copper layer.
 15. A method according to claim 13, whereinapplying a second semiconductor layer to a second surface of the copperlayer comprises laminating the second semiconductor layer to the secondsurface of the copper layer.
 16. A method according to claim 13, whereinapplying a laminate layer to the second semiconductor layer compriseslaminating the laminate layer to the second semiconductor layer.
 17. Amethod according to claim 13, wherein applying an insulation layer tothe first semiconductor layer comprises vacuum pressure impregnating theinsulation layer.
 18. A method according to claim 13, wherein theinsulation layer is an epoxy resin.